Field of the Technology
The present technology is directed to the measurement of the number concentration of airborne particles, to the focusing particles while airborne and to the collection of airborne particles through growth by water condensation. Specifically, it relates to particles in the size range from a few nanometers to a few micrometers in diameter.
Description of Related Art
Most airborne particles are difficult to detect directly because they have diameters smaller than the wavelength of visible light. Often condensational growth is used to enlarge these particles to a size that can be detected optically, thereby providing a means to readily measure airborne particle number concentrations. Condensational enlargement is also used to enable the aerodynamic focusing or collection of particles for chemical or exposure analyses.
Ultrafine particles, with diameters in the nanometer to hundreds of nanometers, are not easily enlarged by condensation. In almost all cases these ultrafine particles must be in an environment of vapor supersaturation before they will start to grow by condensation. Vapor supersaturation means that the concentration is larger than the vapor equilibrium concentration over a flat surface. This enhanced amount of vapor is needed to overcome the particle surface energy associated with its curvature and surface tension.
Hering and Stolzenburg introduced a means to create a supersaturation of water vapor in a laminar flow (U.S. Pat. No. 6,712,881, Hering, S V; Stolzenburg, M R, “A method for particle size amplification by water condensation in a laminar, thermally diffusive flow”, Aerosol Science and Technology 39: 428-436, 2005). Previously, laminar flow condensation methods had used a slowly diffusing species such as butanol as the condensing fluid. The method of Hering and Stolzenburg explicitly accounts for the high molecular diffusivity of water vapor, and achieves growth by water condensation in a laminar flow using a single-stage, warm, wet-walled condenser.
A second laminar flow method for producing small particle growth by water condensation is the “diffusive mixing” approach described by Hering and Lewis (U.S. Pat. No. 7,736,421). This method surrounds the aerosol flow with a warmer, saturated sheath flow in a laminar manner. Once joined, heat and water vapor are exchanged between the two flows by diffusion. Water vapor diffuses into the colder aerosol flow at a slightly higher rate than it is warmed by the surrounding flow, creating a region of water vapor supersaturation within the aerosol flow.